Dual band infrared continuous zoom lens

ABSTRACT

A continuous zoom lens arrangement can image MWIR and LWIR spectral bands to a common image plane. Such an exemplary optical system comprises eight infrared imaging lenses that all transmit over the wavelengths 3.5-11.0 microns and form a collocated image plane for the MWIR and LWIR spectral bands. The lens has six stationary lenses, and two lenses that move in an axial fashion. A cold stop inside the dewar can act as the aperture stop of the system and control the stray light from reaching the FPA. The pupil is reimaged from the cold stop to near the first lens of the system to minimize the size of the lens.

GOVERNMENT INTEREST

The invention described herein may be manufactured, used, sold,imported, and/or licensed by or for the Government of the United Statesof America.

FIELD OF THE INVENTION

This invention relates in general to optics, and more particularly todual-band infrared continuous zoom lens system.

BACKGROUND OF THE INVENTION

With the development of dual band focal plane arrays which image boththe mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR)spectral bands simultaneously at the same image plane, the developmentof various types of optical systems has been required. In order to keepthe same capabilities as a single band system (i.e., MWIR only), it isnecessary to develop similar types of optical systems. One useful typeof optical system is a continuous zoom lens arrangement which enablesFOV changing from the lenses shortest focal length to its longest focallength, and every focal length in between with good optical performance.This has previously been accomplished in both the MWIR and LWIR spectralbands independently. However, a dual band system requires a more complexoptical design because of the need to image both spectral bands at thesame image plane even though this is not the natural tendency of theoptics. These lenses require significant color correction for each focallength. This is very difficult for systems that require significantfocal length changes.

Due to the reduced transmission from the broader spectral band, thefewer optical elements the better in order to reduce ghost images andimprove system noise equivalent temperature difference (NETD).

SUMMARY OF THE INVENTION

An exemplary continuous-zoom dual-band optical lens imager is disclosedbased on infrared optical materials. Such an exemplary continuous-zoomdual-band optical lens imager comprises six stationary lenses; twolenses axially movable; and a focal plane array, wherein said lenses areoptically configured to transmit spectral wavelengths in the range of3.5-11.0 microns to form a collocated image plane for MWIR and LWIRspectral bands.

In another aspect, an exemplary optical operating system is disclosedbased on an optical lens system having an optical lens imager capable ofa variable focal length configured with an exemplary reflective afocalof a like magnification. Said optical operating system comprises thesteps of: light enters the optical lens system via the reflectiveafocal; said light is first reflected off of a primary mirror, whereinsaid primary mirror is a concave parabolic off-axis mirror; said firstreflected light is second reflected from a secondary mirror, whereinsaid secondary mirror is a convex off-axis parabolic mirror; said secondreflected light from said secondary mirror is passed to a fold mirror;said light from said fold mirror is then reflected from a tertiarymirror, wherein said tertiary mirror is an off-axis concave parabolicmirror, and wherein there is an intermediate image plane between thefold mirror and the tertiary mirror; and said light reflected from saidtertiary mirror is reflected off another fold mirror to be passed tosaid optical lens imager of a configured field of view. Said anotherfold mirror is disposed between the optical lens imager and thereflective afocal to achieve a compact optical operating system.

Yet, in another aspect, arm exemplary continuous-zoom dual spectral-bandoptical lens imager is disclosed based on infrared optical materials.Such an exemplary continuous-zoom dual spectral-band optical lens imagercomprises a first lens based on zinc selenide and having an asphericfront surface; a second lens based on germanium and having sphericalfront and back curvatures; a third lens based on barium fluoride andhaving spherical front and back curvatures, wherein the first, secondand third lenses form a triplet; a fourth lens based on zinc selenideand having a spherical first surface and aspheric second surface; afifth lens based on germanium and having a spherical first surface andspherical second surface, wherein said fifth lens is moveably configuredas a first zoom lens to moveably effect a focal length without changingan image plane location for either of the dual spectral bands; a sixlens based on zinc selenide and having spherical first and secondsurfaces, wherein said sixth lens is moveably configured as a secondzoom lens; a seventh lens based on barium fluoride and having sphericalfront and back surfaces; an eighth lens based on zinc selenide andhaving an aspheric first surface and a spherical second surface; and adewar window based on an infrared transmitting material such asgermanium; and a focal plane array. Said dewar window is configured toprovide a vacuum seal to a dewar at cryogenic temperatures. Said focalplane array is either a dual band focal plane array or a broad bandfocal plane array housed in said dewar. An aperture stop or a cold stopis disposed between said dewar window and said focal plane array todetermine a solid angle of the optics for each portion of said focalplane array. A pupil formed by the aperture stop is reimaged in-front ofthe first lens to form an entrance pupil for all fields of view.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional advantages and features will become apparent as the subjectinvention becomes better understood by reference to the followingdetailed description when considered in conjunction with theaccompanying drawings wherein:

FIG. 1 shows an exemplary optical layout of a 3.42× continuous zoom dualband MWIR and LWIR lens imager based on various infrared opticalmaterials.

FIG. 2 shows various exemplary optical layout based on motion of zoomlenses to change the focal length of the 3.42× continuous zoom dual bandMWIR and LWIR lens.

FIG. 3 shows an exemplary graphical representation of motion of zoomgroup 1 (ZG1) and zoom group 2 (ZG2) for continuous zoom dual MWIR andLWIR lens.

FIG. 4 shows an exemplary optical layout of 3.42× continuous zoom dualband MWIR and LWIR lens imager combined with 3.42× reflective afocal,with imager in widest field of view position at fast f/number.

FIG. 5 shows an exemplary optical layout of 3.42× continuous zoom dualband MWIR and LWIR lens imager combined with 3.42× reflective afocal,with imager in field of view position that creates half of the maximumfocal length when combined with the 3.42× afocal thus maximizing thesystem aperture when operating at f/3.0.

FIG. 6 shows an exemplary optical layout of 3.42× continuous zoom dualband MWIR and LWIR lens imager combined with 3.42× reflective afocal,with imager in field of view position that creates the maximum systemfocal length when combined with the 3.42× afocal.

FIG. 7 shows an exemplary optical layout of 3.42× continuous zoom dualband MWIR and LWIR lens imager with 3.42× reflective afocal bypassmirror in position and imager at its longest focal length and at thefastest f/number of the system.

FIG. 8 a shows an exemplary optical path at the maximum system focallength of 445 mm based on an exemplary series of optical configurationsdemonstrating the continuous zoom method of the continuous zoom imagercombined with the 3.42× afocal to create an overall zoom range of 11.7×.

FIG. 8 b shows an exemplary optical path for the focal length at 300 mmamong the exemplary optical configurations of the continuous zoom imagercombined with the 3.42× afocal.

FIG. 8 c shows exemplary imager zoom lenses moved in focal lengths andzoomed toward the focal length of 222.5 mm.

FIG. 8 d shows an exemplary optical path passing through both the afocaland the imager in its widest field of view position to create an overallsystem focal length of 130 mm at f/3.0.

FIG. 8 e shows an exemplary imager configuration at its longest focallength of 130 mm while operating at f/3.0 with the afocal bypass mirrorin place.

FIG. 8 f shows the widest field of view of an exemplary imagerconfiguration in its shortest focal length position, the afocal bypassmirrors being used to form an overall focal length of 38 mm at 173.0.

FIG. 9 shows an exemplary diagram of the change in focal lengths,f/numbers, and entrance pupil diameters for the continuous zoom dualband dual f/number catadioptric optical system.

FIG. 10 shows an exemplary table of optical prescription of the dualband continuous zoom imager enabling the zoom range of 38-130 mmoperating at f/3.0 with a common focus for both the MWIR and LWIR.

DETAILED DESCRIPTION

An exemplary optical system as shown in FIG. 1 comprises eight infraredimaging lenses that all transmit over the wavelengths 3.5-11.0 micronsand form a collocated image plane for the MWIR (3.5-5.0 microns) andLWIR (7.5-11.0 microns) spectral bands. The lens arrangement can havesix stationary lenses, and two lenses that move in an axial fashion. Thelens arrangement is configured as an imager to utilize a cold stopinside the dewar to act as the aperture stop of the system and controlthe stray light from reaching the FPA, as is typically configured in acooled infrared system. The pupil is reimaged from the cold stop to nearthe first lens of the system to minimize the size of the lens. The opticis designed to operate at an f/number of 3.0 and provides a focal lengthrange from 38 mm to 130 mm for a 640×480 element focal plane array withmicron square pixels. This has an equivalent image plane diameter of 16mm, and results in a horizontal field of view range of 5.6-19.1 degrees.This optical system is designed to work with a cold shield height of29.46 mm, although this value could be varied with slight tweaks to theoptical design. The range in focal length result in an overall zoomchange of 3.42×.

Such an exemplary imager can be combined with a reflective afocal ofmagnification 3.42× to achieve a continuous zoom range of 11.6×. Theadditional zoom range can be achieved by using an afocal that utilizes abypass mirror to maintain the imager focal lengths and then combine withthe 3.42× afocal to create focal lengths from 130-445 mm. The entrancepupil of the imager is thus also sized to be 3.42× the size of that ofthe imager by itself. The size of the entrance pupil can be minimizedvia the use of dual f/number capability where the f/number changes tominimize the maxinmum aperture. In this case the f/number will change bya factor of two at the point where the system focal length is half thatof the maximum focal length of the system. The maximum aperture is thusscaled in half.

The optical materials of each lens shown in FIG. 1 can be chosen to notonly transmit the required MWIR and LWIR spectral bands, but to aid inproviding color correction for each spectral band so that the imageplanes will fall in the same location as well provide the same focallength to within a small tolerance. The ray trace for such an exemplaryoptical system is shown in FIG. 1. The first three lenses form a tripletprior to the zoom lenses. The first lens 1 can be a Zinc Selenideelement with an aspheric front surface. This lens is followed by aGermanium lens 2 and a Barium Fluoride lens 3, both of which havespherical front and back curvatures. The fourth lens 4 is a zincselenide lens with a spherical first surface and aspheric secondsurface. The fifth lens 5 is the first of two zoom lenses that move inthe same direction to create the change in focal length without changingthe image plane location for either spectral band. This lens is madefrom germanium and has a spherical first surface and spherical secondsurface. The second zoom lens 6 is made from Zinc Selenide and has bothspherical first and second surfaces. The total travel for the first zoomlens 5 is 43.0 mm and the second zoom lens 6 is 69.9 mm. The last twolenses are comprised of barium fluoride 7, and zinc Selenide 8,respectively. The barium fluoride lens 7 has spherical front and backsurfaces. The last lens 8 has an aspheric first surface and a sphericalsecond surface. The lens is followed by a dewar window 9 comprised on aninfrared transmitting material such as germanium. This window is used toprovide a vacuum seal to the dewar that houses the dual band or broadband focal plane array 11 and maintain it at cryogenic temperatures.Between the window and focal plane array, at an optical distance of29.46 mm is an aperture stop 10 that is maintained at cryogenictemperatures and referred to as a cold stop. This aperture is theaperture stop of the system and determines the solid angle of the opticsfor each portion of the focal plane array. The pupil formed by theaperture stop is reimaged in-front of the first lens 1 to form anentrance pupil for all field of view. The reimaging requires that anintermediate image plane 12 be formed within the lens. This image plane12 falls between the two zoom lenses (5,6) for the majority of zoompositions, but both lenses fall on the object side of the intermediateimage plane for the widest field of view zoom position. There are twolocations for fold mirrors (13,14) which enable the system to be foldedin multiple ways for package purposes if desired. These fold mirrors areoptional and any subset number of mirrors (0,1, or 2) can be useddepending on the desired packaging. The mirrors can also be folded inany 45 degree orientation. The optical rays 15 trace through all of thedescribed lenses and form images at the location of the focal planearray 11. Both the MWIR and the LWIR form its image plane at thislocation for all fields of view.

FIG. 2 demonstrates the path that each zoom lens follows through thevarious zoom positions. In order to change the focal length from theshorter focal length to the longer focal length, the zoom lenses (1,2)move independently away from the image plane and towards the first lensin a smooth continuous motion. Each lens moves independently and at adifferent rate. The prescribed motion is continuous in nature and can bedescribed by this set of curved lines (3,4) as shown in FIG. 2 and FIG.3. The second zoom lens passes through the intermediate image plane toachieve the shortest focal lengths. The range of focal lengths for whichthis occurs are 40-48 mm and result in slightly degraded, albeitacceptable performance. The lens for all other FOV position results in awell corrected, near diffraction limited image for each individualspectral band, MWIR and LWIR. In each case the MTF is well above areasonable value at the Nyquist limit of the detector at 25 lp/mm forboth spectral bands evaluated at the same image plane location. Theoptical prescription for such an exemplary optical system is located inthe exemplary table of FIG. 10. The air gaps between lens 4 and lens 5,between lens 5 and lens 6, and between lens 6 and lens 7 adjust toperform the overall zoom capability. The overall length of the lensremains the same in this process. The curve ZG1 from FIG. 3 representsthe motion of the first zoom lens 1 as shown in FIG. 2. This curve ZG1in FIG. 3 is based on the curve 3 that is drawn in FIG. 2. The lensposition from FIG. 3 corresponds to the distance from this lens 1 to thenext non-moving element as represented by lens number 7, which is lens 5in FIG. 2. At the widest FOV position, the 1st zoom lens 1 is furthestaway from this non-moving lens 5. Likewise, the curve for ZG2 in FIG. 3is based on the motion of the 2nd zoom lens 2 of FIG. 2. The motion ofthis lens is relative to the next non-moving element 5 as seen in FIG.2. The distance between the two curves (ZG1, ZG2 of FIG. 3) representsthe distance between the two zoom lenses (1, 2 of FIG. 2) for any givenfocal length.

This lens can also be used as a multiple FOV lens where the FOV ischanged by an axial zoom and only image at discrete positions along theaxis. Any number of zoom positions could be used from a fixed focallength lens, two-position, to an infinite number of positions.

The re-imaged pupil of the lens is located at a distance of 60 mm infront of the first lens. This enables the lens to be combined with anafocal to change the focal length range of the continuous zoom lens.This change can either be used to create a fixed change in focal lengthrange, or be combined with a switchable afocal to extend the overallzoom range of the system.

The range of continuous zoom with dual band capability can be increasedby the addition of an afocal of the same magnification change as theimager. Such an optical system is shown in, e.g., FIG. 4. The systemcomprises of the imager as previously described combined with areflective afocal, e.g., of the same magnification. To obtain acontinuous zoom, such an exemplary 3.42× zoom range imager is combinedwith an exemplary 3.42× magnification afocal. The new resultingexemplary lens system is described in FIGS. 4-8 where each figurecomprises of the same set of lenses and mirrors, but the position ofsome of the lenses and mirror are moved relative to each other in orderto change the focal length of the system. The continuous movement of theimager lenses allow for a continuous zoom, and the discrete motion ofthe afocal fold mirror to bypass the afocal optical path provides foreither a larger discrete jump in magnification, or enables thecontinuation of the of the zoom range from 3.42× to 11.7×. The discretejump in magnification occurs if the imager is not reset to its shortestfocal length when the bypass mirror is removed from the optical path.The reflective nature of the afocal enables the increase in the zoomrange with no bearing on the chromatic properties of the dual band lens,thus maintaining the dual band zoom capability.

FIG. 4 shows the ray trace of an exemplary imager combined with the3.42× afocal when the imager is in its shortest focal length position.The 38 mm focal length of the imager is multiplied by the 3.42×magnification of the afocal to create a system focal length of 130 mm.This is achieved with the system at an f/number of 3.0. The imagerentrance pupil for this imager FOV position is close enough to theoptimal position for the afocal such that the extent of the traced rays15 will not fall off of the mirror surfaces/The light enters the opticalsystem via the afocal and is first reflected off of the primary mirror16. This mirror is a concave parabolic off-axis mirror. The light isreflected onto the secondary mirror 17 which is a convex off-axisparabolic mirror. This light is passed to a fold mirror 18 and then ontothe tertiary mirror 19 which is also an off-axis concave parabolicmirror. There is an intermediate image plane between the fold mirror 18and the tertiary mirror 19. This re-imaging capability is necessary toalso re-image the pupil that is designed near the primary mirror to forman exit pupil outside of the afocal that is co-aligned with the entrancepupil of the imager that is combined with the afocal. There is anotherfold mirror 20 that lies between the imager and afocal and is used toform a more compact package. The light then passes into the imager thatis configured at its widest field of view position.

FIG. 5 shows the same afocal/imager setup but with the zoom lenses (5,6)moved to a position to create a longer focal length. In thisconfiguration, the focal length of the imager is 65 mm and the combinedfocal length is 222.5 mm. This focal length position is of interestbecause it is the point where the imager and afocal combined with theimager operating at f/3.0, fills up the entire aperture of the afocal.In order to continue the zoom to longer focal lengths, it is necessaryto change the f/number via changing the diameter of the aperture stoplocated within the dewar otherwise known as the cold stop 10. This focallength is exactly one half of the maximum focal length of the system. Bychanging the f/number for all focal lengths greater than the 222.5 mfocal length enables a longer focal length to be utilized for a givenmaximum system aperture size, while maintaining the faster f/number forall other focal lengths to maximize light collection capability. In thiscase, the f/number is scaled by a factor of two for all focal lengthsgreater than 222.5 mm from a value of 3.0 to 6.0. The maximum aperturediameter will then not be violated up to the point where the focallength reaches 445.0 mm, at f/6.0. FIG. 6 shows the afocal/imagercombination where the maximum focal length of 445 mm is achieved. Thisentrance pupil diameter is the same as the entrance pupil for the 222.5mm lens configuration because the aperture stop 10 is resized for anf/6.0, and then reimaged to near the primary mirror 16, minimizing itsoverall diameter. This configuration consists of the imager operating ata focal length of 130 mm as achieved by the movement of the two zoomlenses (5.6).

The shorter focal positions, where the focal length is 130.0 mm and lessare achieved by merely the use of the imager and a bypass mirror 21, asshown in FIG. 7, which keeps the light from entering the magnifiedafocal and directs the light onto the imager. Alternatively, instead ofa bypass mirror, the fold mirror 20 between the afocal and imager couldby flipped in and out of the field of view to enable the imager tooperate without passing through the magnified afocal. FIG. 7 shows oneconfiguration of this circumstance where the imager at a focal length of130 mm and f/3.0 utilizes the bypass mirror to pass light through onlythe imager. Thus, the dual band continuous zoom imager with zoom rangeof 3.42× is allowed to image between the focal lengths of 38-130 mm.FIGS. 8 a-8 f show an exemplary series of all of these positions asstepped through the full zoom range of the system to provide an overallmagnification change of 11.7×. If examined from widest field of view tonarrowest, the fields of view changes from FIG. 8 f to FIG. 8 a. In FIG.8 f, the widest field of view is observed as the imager is in itsshortest focal length position and the afocal bypass mirrors are used toform an overall focal length of 38 mm at f/3.0, FIG. 8 e shows the nextsignificant position where the imager is at its longest focal length of130 mm while operating at f/3.0 with the afocal bypass mirror in place.All of the focal length positions between these two values are achievedas previously discussed. To continue the zoom, the afocal bypass mirroris moved out of the way and the imager zoom lenses are moved back to theoriginal position of where they achieved the 38 mm focal length. Theoptical path now passes through both the afocal and the imager in itswidest field of view position to create an overall system focal lengthof 130 mm at f/3.0 as seen in FIG. 8 d. The imager zoom lenses are thenmoved to longer focal lengths and zoomed up to the point where the focallength equals 222.5 mm as shown in FIG. 8 c. At this point, in order tocontinue zooming to longer focal lengths, it is necessary to change theaperture stop diameter to maintain the entrance pupil smaller than 74.1mm. The f/number is scaled by a factor of two by changing the diameterof the aperture by the same amount. The f/number thus changes to 6 andthe diameter of the aperture is cut in half. The lens can then continueto zoom from 222.5 mm to 445 mm without violation of the maximumaperture constraint.

FIG. 8 b shows the optical path with the focal length at 300 mm and FIG.8 a shows the optical path at the maximum system focal length of 445 mm.Both of these are at an f/number value of 6. The entrance pupil diameterof this narrowest field of view matches that of the entrance pupildiameter at half its field of view. The diagram in FIG. 9 shows thevarious focal lengths, f/numbers, and aperture values for eachcorresponding focal length position. Any position where the aperture issmaller than 74.1 mm at f/3 is operated at f/3.0. All other positionsoperate at f/6.0. Other f/number combinations are feasible. It is alsonot necessary to change the f/number at all if a larger aperture isacceptable, but the dual f/number approach enables the maximization offocal length versus minimization of aperture size.

It is obvious that many modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that within the scope of the appended claims, theinvention may be practiced otherwise than as described.

What is claimed is:
 1. An optical operating system based on an opticallens system having an optical lens imager capable of a variable focallength configured with a reflective afocal of a like magnification, saidoptical operating system comprising the steps of: light enters theoptical lens system via the reflective afocal; said light is firstreflected off of a primary mirror, wherein said primary mirror is aconcave parabolic off-axis mirror; said first reflected light is secondreflected from a secondary mirror, wherein said secondary mirror is aconvex off-axis parabolic mirror; said second reflected light from saidsecondary mirror is passed to a fold mirror; said light from said foldmirror is then reflected from a tertiary mirror, wherein said tertiarymirror is an off-axis concave parabolic mirror, and wherein there is anintermediate image plane between the fold mirror and the tertiarymirror; and said light reflected from said tertiary mirror is reflectedoff another fold mirror to be passed to said optical lens imager of aconfigured field of view, wherein said another fold mirror is disposedbetween the optical lens imager and the reflective afocal to achieve acompact optical operating system.
 2. The optical operating systemaccording to claim 1, wherein a pupil near the primary mirror isreimaged to form an exit pupil outside of the afocal that is co-alignedwith the entrance pupil of the imager that is configured with theafocal.
 3. The optical operating system according to claim 1, whereinsaid optical lens imager comprises: a plurality of stationary lenses;two zoom lenses axially movable to effect a variable focal length of afocal length position; and a focal plane array.